There are numerous applications today in which automatic control of lighting in a home is useful and desirable. For example, a common use of such a timed device is to create the appearance that the house is occupied when in fact the residents are away on vacation. In a number of these applications, an electromechanical device or an electronic device is used to provide a switching action that connects a lamp to the AC supply at one time and disconnects the lamp from the AC supply at another time. In some such systems, the switching action is initiated by a timer, and in other such actions the switching action is initiated by a light sensor.
A commonly encountered difficulty with the systems in which light sensing is used is that the sensitivity of the light sensor must be adjusted in accordance with the environment in which the device is installed. Moreover, in such applications, the presence of clouds or other obstructions of the sum can cause the lamp to be switched on at an inappropriate time of day. Also, in some such applications, the presence of a second source of artificial light can interfere with the desired operation of the lighting control system. For these reasons, among others, the use of a timer instead of a light sensor is often preferred.
The timed switching systems known to be available for the control of home lighting are designed to enable the user to "program" the times at which the lamp is switched on and off. In some devices, an analog input is provided for, with an analog display of the programmed time. In other devices, a digital keypad is provided for the entry of programmed times, and a digital display is provided. A common problem encountered in the use of all such devices is the complexity of the programming procedure. For many people, the programming procedure is too difficult to learn. For others, the procedure is too difficult to remember from one use of the device to another. Even those who can learn and remember how to use such a timing device often find it inconvenient to use because of the time required for the programming operation and the inconvenience of having to program the device several times during the year to match the changing times of dusk during the year. A common experience is for the resident of the house to think of the timer as he is preparing to leave the house, in a rush to depart on his vacation and already burdened with other last minute concerns.
Another undesirable feature of the timed switching systems is that if there is a power failure or if the system is disconnected from the alternating current supply, the timing information stored in the system is lost, except in a few systems in which a battery backup is provided.